42 CFR 84.1137 - Inhalation and exhalation valves; minimum requirements.
(a) Inhalation and exhalation valves shall be protected against distortion.
(b) Inhalation valves shall be designed and constructed and provided where necessary to prevent excessive exhaled air from adversely affecting filters, cartridges, and canisters, except where filters of dust, fume, and mist respirators are specifically designed to resist moisture as prescribed in § 84.1145.
(c) Exhalation valves shall be:
(1) Provided where necessary;
(2) Protected against damage and external influence; and
(3) Designed and constructed to prevent inward leakage of contaminated air.